Power line carrier frequency telephone system



Dec. 2, 1941. D. MlTCHE LL POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEMFiled Oct. 22, 1940 b Oh WE w mi 2 w Z @W 05 E /H &N mm M V/ WM. w

Patented Dec. 2, 1941 POWER LINE CARRIER-FREQUENCY TELEPHONE SYSTEMDoren Mitchell, Bound Brook, N. J., assignor to Bell TelephoneLaboratories,

Incorporated,

New York, N. Y., a corporation of New York Application October 22, 1940,Serial No. 362,249

4 Claims.

This invention relates to electric wave transmission systems, and, moreparticularly, to a power line carrier frequency telephone system.

Carrier frequency telephony on transmission lines primarily intended forthe transmission and distribution of low frequency high voltage electricpower has, of course, been known for some time although never veryextensively applied. It has, however, become of increasing interest inView of the somewhat extensive electrification of rural areas and thepossibility provided by the rural power lines of providing telephoneservice thereover to the scattered inhabitants of the sparsely settledregions through which the power line extends.

An object of this invention is to provide an improved arrangement fortransmitting and receiving telephonic communication on high frequencywaves over a transmission path, such as a high voltage power line.

In accordance with the invention, a plurality of telephone stations anda central office for the stations are coupled to a power line.Transmitting from each station is on a common high frequency carrierwave and receiving at each station is on a second common high frequencycarrier wave. The central oifice acts not only as a means for connectingparties on the power line telephone system with an outside or generaltelephone system, but constitutes a relay or frequency change-overstation for the telephone stations of the power line system, that is,the outgoing transmission from a telephone station on the one highfrequency carrier wave is received at the central office, and, aftersuitable demodulation, the modulation products are remodulated onto thesecond high frequency carrier wave for transmission to the called orlistening telephone station. No voice-operated devices are necessary atthe central ofiice although voice-operated devices are employed at thetelephone station to enable the transmitting circuit thereat. Whenvoice-operated devices for enabling the transmitting circuit areprovided at the telephone stations and the latter are arranged fortransmitting on the same carrier wave, intentional or inadvertentoperation of the voice-operated devices at a called station while theparty at the calling station is talking may produce an unpleasant orundesirable interruption or interference with the transmission from thecalling station.

A feature of this invention is the provision at the telephone station ofmeans in the receiving circuit to disable the transmitting circuitenabling means while transmission is incoming to the receiving circuit.

A more complete understanding of the invention and the various featuresthereof will be obtained from the detailed description which followstaken in conjunction with the appended drawing, the single figure ofwhich illustrates an embodiment of the invention.

The Single figure of the drawing shows a power line carrier frequencytelephone system comprising a power line I00 and a plurality oftelephone stations A, one of which is shown in detail, and a centraloffice C0 for the stations coupled to the power line. The system isarranged so that the stations communicate with one. another, or with ageneral telephone system through the central ofiice. Transmitting, ortransmission outgoing from each station, is on a common high frequencyor carrier wave F1, and receiving, or transmission incoming to eachstation, is on a second common high frequency or carrier wave F2, thecentral ofiice constituting a relay or frequency change-over station forreceiving the transmission of the calling or talking party on the onefrequency and for converting it to the frequency on which the called orlistening part is able to receive it. e

The power line has one wire grounded and is adapted to transmit lowfrequency, high voltage electric power simultaneously with its use as atransmission line for carrier frequency telephonic communication. Thepower line, of course, could'be an insulated single phase line, orcomprise a pair of phase wires of a multiphase line, or other type ofpower line.

Each telephone station A comprises a transmitting circuit In, areceiving circuit H and a coupling line l2. The transmitting circuitcomprises a transmitter I'3, a volume control amplifier M, a modulatorIS, a high frequency or carrier wave generator or oscillator l6, anoutput amplifier I! and a band-pass filter IS. The transmitter I3, forexample, of the carbon granule type, is connected to the volume controlamplifier through a transformer l9, and is connected in series with onetransformer winding and a source 20 of talking current. The volumecontrol amplifier may be of the voice-operated gain adjusting or volumelimiter type, or may be provided with a manual control to provide" fortalkers of different transmission levels so that the voice current levelinto the modulator will be the same at all times, or to insure thatthere will be a high ratio between the communication current and thenoises in the transmission line, The

modulator may be of any suitable type, as may the oscillator I 6. Theoscillator generates a high frequency F1 which, during transmitting, issupplied to the modulator and is modulated by the voice frequencycurrents impressed on the modulator from the amplifier M. The modulationproducts are amplified in the amplifier l! and either the upper or thelower side-band, or both side-bands of the modulated high frequency waveare passed by the filter Hi to the line. The filter is adapted totransmit a band of frequencies comprising the upper or the lowerside-band or both side-bands of the modulated high frequency wave foroutgoing transmission from the station, i. e. it transmits the band F1to Fri-F3, F1 to F1F3 or F1 to FliF3, where F3. is the upper limit ofthe audio frequency band beingtransmitted in the system. Normally,however, no high frequency wave is delivered to the modulator since theoscillator is short-circuited by the normally closed contact 2| of relay22-. The nor mal condition of the station would be that in which thesubscriber is not talking. An amplifier-detector 23 is connected acrossthe output of the amplifier i4, and is adapted, when sound waves areimpressed on the transmitter l3, to amplify and detect a portion of thevoice frequency currents being delivered to the modulator and to deliversuch detected currents to the relay 24 to cause it to close its normallyopen contact 25.

One end of the winding of relay 22 is connected through the normallyclosed contact 26 of the signaling key 21 to the junction of aresistance 53 associated with the contact 25 and a condenser 28, oneterminal of wihch is connected to ground. The other end of the windingof relay 22 is connected to ground through a relay 29, a resistance 30and a source 3| of current. Another resistance 32 is connected betweenthe positive terminal of source 3| and the normally open contact 33 ofkey 21. An interrupter 34, to vibrate, for example, at 20 cycles persecond, is connected for operation on closure of normally open contacts33 and 54 of the key 21.

The receiving circuit comprises a band-pass filter 35, an amplifier 36,a second amplifier 31 for additional amplification if desired, ademodulator 3'8, station signaling or ringing means 39', an attenuatornetwork 40 and a. telephone receiver 4|. The filter is adapted totransmit a band of frequencies comprising the upper or lower side-band,or both side-bands, of the modulated high frequency wave employed in thesystem for transmission incoming to the station, that is, the filterpasses the band F2 to F2+F3, F2 to Fz-Fs or F2 to Fzi-Fs, where F2 isthe carrier wave frequency for receiving and F3 is the upper limit ofthe audio frequency band being transmitted in the system. The amplifier36 may comprise an electron discharge device whose input control gridbiasing potential is determined by the potential drop in the cathoderesistor 42, and in the resistor 43, which is supplied with the directcurrent component of the output of the demodulator 38. Variation in theoutput level of the demodulator from a preassigned level varies thevalue of the current fed back to the resistor 43 through the connections41, to vary the bias on the control grid of the discharge device tochange the gain of the control amplifier and to restore the demodulatoroutput to its preassigned level. The variation in the modulator outputmay result from change in the level of the transmission incoming to thestation because of different distances between any one station and theother stations coupled to the power line. The resistance 44 andcondenser 45 act with retard coil 48 and the winding of relay tosuppress alternating current feedback in the controlled amplifier. Thedemodulator is shown as comprising a plurality of variable resistancedevices such as copper-copper oxide rectifier units in bridgeconfiguration. As already indicated, the gain control resistor 43 isconnected across the terminals 46 of the demodulator through connections41, a retard coil 48 being provided in one connection. Condenser 49 is ablocking condenser. The station signaling means 39 may comprise a ringeradapted to be operated, for example, on a 20-cycle ringing current. Thenetwork 48 compensates for the lower volume level required for listeningas compared with that required for operation of the signaling means 39.A relay 50 is included in the feedback connection from the demodulator,and has a normally open contact 5! associated therewith closure ofwhich, while normally closed contact 52 of relay 29 remains closed,short-circuits the input to the amplifier-detector 23. Relay 50,however, will be energized only when transmission is incoming to thestation. The object of this arrangement will be pointed out in moredetail hereinafter.

The station is connected to the ower line by the coupling line [2, oneconductor of the latter being connected directly to the grounded wireand the other to the non-grounded wire through a coupling condenser 56.With an insulated single phase line, or in case of connection to a pairof phase wires of a multiphase system, a coupling condenser would beprovided in each conductor of the coupling line. Coupling might also beaccomplished by use of a power distribution transformer, for example inthe manner described and claimed in L. K. Swart application, Serial No.359,879 filed' October 5, 1940. In any case a suitable protector 55 isprovided to safeguard the subscriber and the station equipment againstthe hazard of the high voltage on the power line.

The central ofiice will now be described. It comprises an incoming orreceiving circuit and an outgoing or transmitting circuit 10 between thecoupling line l2 and the operators line jack 6|. The incoming circuit 60comprises a filter 62 similar to filter H!- of the telephone station, anamplifier 63, a demodulator 64 with a suitable automatic volume controlfeedback connection 65 from, the demodulator to the amplifier, a relay66 adapted to respond to the low frequency signaling current, forexample, 20 cycles per second originating at a telephone station, and anattenuator network 61 to equalize for the different levels required forthe signaling current and for the voice frequency currents. The.armature associated with the relay 66 is connected to ground through acurrent source 68, and its normally open contact 69 is connected toground through the winding of relay H and through the operators signallamp 12. The outgoing circuit 1.0 comprises an amplifier 13 similar tothe arnplifier f4, a modulator M, an output amplifier I5 and a band-passfilter 16 similar to filter 35. An oscillator Tl generating the highfrequency carrier wave F2 is coupled to the modulator, the connectionsincluding the normally closed contact 18 of an interrupter or buzzer 19adapted to respond to a low frequency current, for example, 20 cyclesper second, to be supplied over the connection 8!) through the normallyopen contacts 81 of relay H when the latter is energized ,receiver 4|would be supported on aswitchhook which, as will be evident to thoseskilled in the art, could have associated therewith normally opencontacts controlling the talking current circuit of the transmitter I3and the power supply circuit for the oscillator and the transmittingcircuit amplifiers. As shown in the drawing, the receiver has beenremoved from its hook and the talking current circuit for thetransmitter I3 is closed and the oscillator and the amplifiers of thetransmitting circuit are understood to be connected to their powersupply circuits. The receiving circuit amplifiers are continuouslyenergized o that the receiving circuit is at all times in condition toreceive incoming transmission. By listening for a moment, the callingparty ascertains if the power line is already in use. If it is idle, hedepresses the key 21, the number of times and for a period specified forthe called station by the signaling code of the power line telephonesystem. Each time the key is depressed, energizing current is suppliedfrom source 3| to the interrupter through contact 33, and energizingcurrents for relays 22 and 29 are supplied from the same source, thecurrent circuit for the relays being completed by closure of contact 54.Operation of relay 34 causes the contact 2| of relay 22 tobreak-and-make at the interrupter frequency, for example, 20 cycles persecond. This modulates, at a corresponding frequency, the carrier waveF1 supplied to the modulator from the oscillator l6, and the modulatedcarrier wave is transmitted through amplifier I1 and filter l8 to theline and over the latter to the central office and through the centraloffice to all of the stations. The modulated carrier wave is transmittedby the filter 62 to the amplifier 63 and is demodulated by thedemodulator 64, the demodulated signal energizing the relay 66. Closureof the normally open contact 69 energizes relay H and the signal lamp12. Operation of relay 1| closes the normally open contacts 8| and lowfrequency signaling current is supplied over connections 80 to thetransmitting circuit. The interrupter I9 is operated thereby andmodulates the carrier wave F2 by the corresponding break-and-make ofcontact 18. The modulated carrier wave is amplified in amplifier l5, andtransmitted through the filter 16 to the power line. The modulatedcarrier wave F2 is received at all of the stations, being passed byfilter 35 to the amplifiers 36 and 31 and demodulator 38. Thedemodulated signal is delivered to and operates the signaling means 39.When the called party answers, the calling party talks into thetransmitter I3, and the voice frequency currents developed in thetransformer are amplified in amplifier l4 and impressed on themodulator. Simultaneously the voice currents are impressed on theamplifier-detector 23. The rectified voice current output of theamplifier detector energizes the relay 24, and normally open contact 25closes. The condenser 28, normally under charge from source 3 I,discharges through resistance 53, and relays 22 and 29 energize to openthe normally closed contacts 2| and 52 for as lon as the calling partycontinues to talk. The carrier wave F1 supplied to the modulator ismodulated by the voice frequency currents, and the peechmodulated'carrier wave is amplified in amplifier I! and transmitted tothe line through the filter I 8. The modulated carrier wave enters thecentral ofiice and is transmitted through the filter 62 through theamplifier 63 and demodulator 64. A preassigned output level from thedemodulator is maintained through the automatic volume controlconnection 65. The demodulation voice frequency products pass throughthe network 61 and the normally closed contacts 82 of relay H to theoutgoing circuit 10. The voice frequency currents are amplified inamplifier l3, modulated onto the carrier wave F2. and are transmittedthrough the output amplifier 15 and filter 16 to the power line. Themodulated carrier wave travels over the power line to the called stationand enters the receiving circuit H thereat. The modulated wave istransmitted by the filter 35 to the amplifiers 36, 31, and demodulatedby the demodulator 38. The demodulated voice frequency currents aredelivered to the receiver 4| through network 40. The direct currentcomponent of the demodulation products is fed back through theconnections 41 to the gain control resistor 43 to maintain thedemodulator output level at a preassigned value. At the same time, thefeedback current energizes the relay 59 and causes closure of thenormally open contact 5|. Since at the called station, relay 29 wouldnot be energized, contact 52 is closed and the amplifier detectorcircuit at the receiving tation is short-circuited or disabled. Thecalled or the listening party cannot intentionally 'orv accidentallytake control of his transmitting circuit by talking, or perhapscoughing, into his transmitter while transmission is incoming throughthe station. If the called or listening party were free to use histransmitting circuit, it is apparent that interference with transmissionfrom the other station would result because of the presence on the powerline of outgoing transmission from two stations on the same carrierwave. Since the receiving circuits of all of the stations in the systemare normally energized when any two stations, or any one station and thecentral office have established a connection, the transmitting circuitsin each of the other stations will be disabled during the talking periodof such connections.

If the calling party had desired to call the operator at the centralofi'ice instead of any telephone station on the line, the procedurewould have been the same as described above; the operator responds tothe central office signal by plugging her telephone set (not shown) intothe jack 6|. The operator, therefore, receives on carrier wave F1 andtransmits and signals on carrier wave F2. The operator may connect apower line telephone station and a party on an outside or generaltelephone system through conventional switchboard facilities.

The system described with reference to a power line transmission path isapplicable also to a transmission path provided by a pair of telephonewires. If the line I00 comprised two conductors balanced with respect toground instead of having one grounded as shown in the drawing, and acondenser 99 were inserted as indicated, such an arrangement would berealized. This would permit private or semiprivate service to beprovided over an ordinary rural telephone line.

What is claimed is:

1. A carrier frequency telephone system comprising a telephone stationcoupled to a line, said station comprising a. transmitting circuit fortransmittinglon a high frequency- :electric wave,

an=amplifier for an incoming modulated highfrequency wavey' ademodulator for demodulating the amplified modulated high frequencywave, i a feedback connection from said demodulator -tosaid-amplifier,and means insaid feedback con nectibn fordisabling thetransmittingcircuit en abling -means during receiving by said receiving2. "A' carrier frequency telephone systemcomprising a telephone-stationcoupled to a 1ine,--saidstation comprising a transmitting circuit fortransmitting -on a-high frequency electric Wave; and areceiving circuitfor receiving on another high-frequency electric wave, said transmittingcircuit being-normally disabled andincluding voiceenergy operated meansfor enabling it, said receivingcircuit being normally enabled andincluding-anamplifier for an incoming modulated-:- high frequency waveyademodulator for demodu-- latingthe amplifieclmodulated high frequencywave means for disabling the transmitting circuit enabling means duringreceiving by said re-- ceiving' circuit, and means for feeding'backanoutput-component of said demodulator-to regu-- late the gain ofsaid amplifier and-to operate-said disabling meanse 3; IA carrier 2frequency: telephone :system com-' prising a telephone station coupledto aazline, said station: comprising: a. transmitting circuit Y fortransmittinggvon; a high; frequency electric wave,

2 saidrtransmittinga circuit being normally disabled.andincludingmeansito enable it for transmission v to th'eilirien and: a:receiving circuit for vreceiving onitanother :high frequency :electricwave, said re- 1 ceivingcircuitbeingcontinuously enabled andincludingaanxamplifier for modulated high frequencyeleotric waveszincoming to lthe station fromathe: line; a ifeedbackwconnection fromthe.

outputricircuit :ofJsaidamplifier to its input circult, and meansin;said;;feedbackl connection for ;disablingdaheztransmitting; circuit.enabling means duringireceivingby said receiving circuit.

4. A carrier;frequencyatelephone system comprising .a;.telephone stationcoupled to a line, said station; comprising a r transmitting circuit fortransmitting aongaehighflfrequency electric wave,

saidctransmittingmircuit being normally disabled and: includingimeans toenable it. for transmission toi'th'e line, :andra, receiving circuit.for receiving on; another high vfrequency electric .Wave, saidreceivingicircuit :being :continuously enabled and. inoluding :an;amplifierwforqmodulated high. frequencyelectric wavesincoming from theline, an

amplifier gain: control. feedback connection from the output circuit ofsaid: amplifier to its input -circuitr, and means said feedbaclcconnection i for disabling the transmitting circuit enablingmeansduri-ng receiving by-said receiving circuit.

DOREN MITCHELL.

